Here you can find a video explaining how to create a foldable paper bag from a square paper sheet. In this video I used a A3 paper sheet (29.7cm x 42cm) but you can use it for any size.

Here is a template with foldable lines that you can print (click on it).

Here is a template with foldable lines, fold directions and sizes that you can print (click on it).

If you want more ready templates you can find some in this catalog (they seem to be free).